Methods have become known in recent years for the production of heavy molecular ions of organic substances, all of which have the disadvantage that the ions have a high average initial velocity which is the same for ions of all masses. In addition, there is a wide spread of initial velocities. The resulting ion beam fills a wide phase space and is difficult to use with conventional mass spectrometers.
More particularly, the production of ions by generating ultrasound or acoustic shock waves on the surface of solid matter was predicted some considerable time ago and is described in detail in printed German patent specification DE-PS 27 31 225. For purposes of this invention, the sound range from approximately 10.sup.9 to 10.sup.13 Hertz is referred to as "hypersound".
A phenomenon was recently discovered by L. N. Grigorov in which molecules in ionized form are shaken off the surface of a thin foil when the foil is bombarded with a laser pulse on the reverse side. This method is suitable for generation of ions from extremely large molecules in the order of magnitude of 1,000,000 Daltons. The method is described in detail in L. N. Grigorov, Bulletin of the USSR Academy of Science, Dept. of Physical Chemistry, v. 288, p. 654, 1986 (experimental setup), v. 288, p. 906, 1986 (theory) and v. 288, p. 1393 (shaking off the ions).
The theory put forward by Grigorov explains this effect by the amplification of a stationary hypersonic wave in the foil by stimulated emission of hypersound in a thin-layered field of considerable electronic excitation near the reverse surface. This effect, described by Grigorov as an "acoustor", resembles the amplification effects of microwaves and light by MASER and LASER (microwave amplification or light amplification by stimulated emission of radiation). The considerable electronic excitation of the very thin field is produced by a pumping effect of the laser pulse in the electronic states of the solid matter.
The hypersonic waves generated by the effect have frequencies of approximately 10.sup.11 Hertz. Molecules are vigorously shaken off by the considerable intensity of the longitudinal hypersonic waves passing transversely through the foil. The ions are ejected in an outwardly neutral plasma consisting of electrons and ions, more than 99% being ionized by a single charge according to estimates by Grigorov.
Irrespective of their mass, all molecules gain approximately the same acceleration from the shaking process and leave the surface with approximately the same average velocity of about 5,000 meters per second. Although the average velocity is the same, the spread of individual velocities is very large, varying from one third to three times the average velocity. Since the spread of energy corresponds to the square of the spread of velocities, the spread of energy between maximum and minimum energy for the particles of a particular mass amounts approximately to a factor of 100. Particles of various masses therefore have mass-proportional average energy.
In comparison to the length of the laser pulse, the shaking-off process lasts a relatively long time. With a pulse length of approximately 10 microseconds from a neodymium YAG laser operating without a Q-switch, the shaking-off of ions could be observed for approximately 1 millisecond with exponential decrease after the laser pulse was terminated. With this method, molecules are essentially transferred whole from the surface to a free-flying ionized state, with no observable limit apparently placed on the magnitude of the molecules. There are indications that ions up to a magnitude of 2,000,000 Daltons can be ionized whole with this method.
Another known method of ion generation is the production of whole molecular ions of high-molecular substances by matrix-assisted laser desorption. This method is described in general in "Matrix-Assisted Laser Desorption/Ionization Mass Spectrometry of Biopolymers", F. Hillenkamp et al., Analytical Chemistry, v. 63 p. 1193, 1991.
In accordance with this method, the molecules of the substance under examination are dispersed in a suitable organic substance (called a "matrix") and applied to a suitable base, for example a level surface on the end of a metal insertion rod. A brief focused laser light pulse lasting less than 10 microseconds (generally only 10 nanoseconds) applied to the substance/matrix mixture then produces a plasma cloud which, with a suitable matrix, consists of a mixture of essentially neutral matrix molecules and singly charged ions of the substance under examination.
With this method, the molecules of the substance under examination are for the most part transferred whole to a free-flying ionized state with no observable limit apparently placed on the magnitude of the molecules which can be ionized. Ions up to a magnitude of 300,000 Daltons have already been ionized whole with this method.
According to more recent examinations reported by R. B. Beavis and B. T. Chait, Chemical Physics Letters v. 181, p. 479, 1991, the ions in the quasi-exploding and, at the same time, adiabatically cooling plasma cloud are accelerated by friction with the matrix molecules. In so doing, all ions of large masses gain approximately the same average velocity of about 750 meters per second with a distribution of individual velocities varying from approximately 300 meters to 1,200 meters per second.
Both of the above-described methods have problems when used with conventional mass spectrometers. Time-of-flight mass spectrometers, which accommodate the pulsed production of ions, have so far been used with these ionization methods. On closer examination, however, time-of-flight mass spectrometers do not allow optimal results to be achieved for several reasons. More particularly, for use with a time-of-flight mass spectrometer, the ions must undergo a twofold filtration process: firstly, time filtration in order to obtain only ions from a small time window of just a few nanoseconds, and secondly, energy filtration in order to make the time-of-flight principle applicable. In addition, the ions have to be focused from a widespread phase space to a narrow phase space which, according to Liouville's theorem, is not possible with optical means.
For example, for his experiments with the laser-induced hypersound ionization method, L. N. Grigorov used a time-of-flight mass spectrometer with a Mamyrin reflector for focusing energy, and an inline energy filter. However, if an ion production period of only 100 microseconds is assumed for hypersonic production of the ions and a time window of 10 nanoseconds is taken as the time-of-flight window, only 1/10,000 of the ions produced remain usable.
Even with a time-of-flight mass spectrometer used with an energy-focusing Mamyrin reflector, focusing of energy is limited to approximately 1% of the flight energy, from which there is a further reduction to a maximum of 1/100 of the ions. The maximum usable proportion of the ions in a time-of-flight spectrometer is therefore one millionth of the total ions formed, even neglecting focusing losses of an unknown magnitude.
In addition, the laser-induced hypersonic method of ion production has a further serious drawback. At a velocity of approximately 5,000 meters per second, a singly charged ion of 2,000,000 Daltons has a kinetic energy of approximately 0.5 million electron volts. Ions with this energy can no longer be handled in a mass spectrometer of normal dimensions since fields of exceptional intensity would have to be used for focusing and deflection. Present laboratory mass spectrometers operate with maximum ion energies of approximately 50 kev.
The matrix-assisted ionizing laser desorption method described above has similar drawbacks. Although both the time window for formation and energy spread are more favorable in this instance, the divergence and thus the focusability of the ion beam, which is formed by the expanding plasma cloud, is much more disadvantageous. The phase space (customarily formed from local coordinates and velocity coordinates) is also therefore very large and unsuitable for mass spectrometry. Here too, solely time-of-flight mass spectrometers have so far been used.
Consequently, it is the task of the invention to find a method of making ions of large organic molecules, which are produced at high velocities in a widespread phase space, accessible fully and with high efficiency for mass-spectrometric examination.